1. Field of the Invention
The present invention relates to a method and an apparatus for controlling an internal-combustion engine in which the ignition timing and the air-fuel ratio are subject to feedback control to respectively take optimum values so as to operate the engine with the best specific fuel consumption.
2. Description of the Prior Art
Conventionally, the ignition timing in an internal-combustion engine is adjusted by controlling, for example, the number of engine revolutions, the intake manifold pressure, etc., in accordance with the running condition of the engine, such that the engine output is made maximum and at the same time the specific fuel consumption is made minimum so long as there is no particular reason such as knocking, such as a problem in exhaust gas characteristic, etc. In such a conventional method, it has been difficult to always maintain the control accuracy high because of the variations in individual engines, changes in environment or the like and it has been impossible to avoid losses to some degree in engine output and in specific fuel consumption.
The air-fuel ratio in an internal-combustion engine is set to a value of theoretical (stoichiometric) air-fuel ratio or leaner than it in the viewpoint of fuel consumption in the normal running condition of the engine, to a high value (about 13) for the maximum output in acceleration or in climbing slope in which the accelerator is largely opened, and to a value taking the stability or the like in idling. Consideration will be now given only with respect to the air-fuel ratio control in the normal running condition. In the conventional carburetor, open loop control has been performed and some degree of loss in specific fuel consumption has been unavoidable due to the variations in individual engines, the aging of the engine, the variations in producing carburetors, or the like. Further, in an electronically controlled fuel injection device in which the intake air quantity in an engine is measured by an intake air sensor, so as to calculate a desired quantity of fuel by a computer or the like to thereby inject a desired quantity of fuel into an intake pipe through an electromagnetic valve in accordance with the calculated value, a closed loop control has been employed in practical applications in which the control direction of the stoichiometric air-fuel ratio (about 15) is detected by an oxygen concentration sensor provided in an exhaust pipe so as to correct the desired fuel quantity. Also in the carburetor control, a closed loop control has been received in some applications in which the air quantity at an air bleed is sensed by the above-mentioned oxygen sensor so as to detect the control direction of the stoichiometric air-fuel ratio to thereby correct the fuel quantity. In fact these closed loop controls are effective in correcting variations in the air-fuel ratio. However, the stoichiometric air-fuel ratio is not the air-fuel ratio which optimizes the specific fuel consumption and therefore there occurs a loss in the fuel consumption.
A control method in which such a loss in fuel consumption is eliminated to optimize the specific fuel consumption is known, for example, by U.S. Pat. No. 4,026,251. In the method, the air bypassing the carburetor is subjected to dither (that is, the air-fuel ratio is repeatedly changed between rich and lean values at a predetermined frequency) to detect the control direction of the air-fuel ratio to make it possible to improve the fuel consumption so that the air-fuel ratio is corrected by an auxiliary air valve bypassing the carburetor. In this method, particularly, an engine causes one revolution at each of two reference air-fuel ratios (a relatively rich air-fuel ratio and a relatively lean air-fuel ratio) and the number of engine revolutions NeR in the rich air-fuel ratio running state is compared with the number of engine revolutions NeL in the lean air-fuel ratio running state, whereby control is made such that when NeR&gt;NeL the bypassing air is decreased, while when NeR&lt;NeL the bypassing air is increased.
In the case where a change in engine output is detected on the basis of the number of engine revolutions, it is important to know the cause of the change in the number of engine revolutions because it may change due to various causes. The conventional method as mentioned above has no means to detect whether a change in the number of engine revolutions has been caused due to a change the air-fuel ratio or due to any other external cause (such as accelerator actuation, going up/down slope, etc.) and therefore there may be a possibility to further deteriorate the fuel consumption by causing control in the direction opposite to that in which the fuel consumption can be improved.
Now, the following description will be turned back to the control of ignition timing. A method in which the ignition timing is feedback-controlled in order to eliminate the above-mentioned loss to thereby make it possible to cause an engine to operate with its maximum output is known, for example, by U.S. Pat. No. 3,142,967. In this method, an engine is made to run with each of two different ignition timings in the vicinity of a desired ignition timing, and the number of engine revolutions Nr, when the engine runs with a relatively retarded one of the two different ignition timings, is compared with the number of engine revolutions Na, when the engine runs with the other relatively advanced ignition timing of the two to thereby correct the desired ignition timing such that when Nr&lt;Na the desired ignition timing is further advanced by a predetermined value while when Nr&gt;Na the desired ignition timing is retarded by a predetermined value so as to obtain an optimum ignition timing which may provide a maximum engine torque.
In the case where a change in engine output is detected on the basis of the number of engine revolutions, it is important to know the cause of the change in the number of engine revolutions because the revolution number may change due to various causes, as already discussed above. The method as disclosed in the above-mentioned U.S. patent does not detect and determine whether a change in the number of engine revolutions has been caused due to the ignition timing or due to any other external cause (such as accelerator actuation). Accordingly, in such a system there is the possibility that a correction control for the ignition timing, especially in an acceleration/deceleration operation, in coming up/down a slope, etc., will be made in the direction opposite to the optimum ignition timing which may provide a maximum torque, thereby resulting in a reduction in the number of engine revolutions lowering the engine output and deteriorating the fuel consumption. To eliminate this defect, an internal-combustion engine control method has been proposed in which the optimizing control for the ignition timing and the optimizing control for the air-fuel control are made alternately to control the ignition timing as well as the air-fuel ratio so as to always optimize the fuel consumption without being affected by any external cause such as accelerator actuation. Namely, a best ignition timing is searched through the control to increase the specific fuel consumption and then a best air-fuel ratio is searched through the control to further increase the fuel consumption. In the method, however, the response time is poor since it takes a long time for the control because the ignition timing optimizing control and the air-fuel optimizing control are alternately performed. Furthermore, when the operating condition of the engine changes frequently there may occur a problem in the reduction of fuel consumption in some cases where it becomes difficult to control the ignition timing and the air-fuel ratio to their optimum values.